The long-term objective of this application is to understand the role of the host immune responses in the progression of mammary neoplasia. The specific aims focus on two approaches to this objective: (1) characterization of the host responses that accompany the progression of preneoplastic hyperplastic alveolar nodules and relatively benign mammary tumors to frankly neoplastic, metastatic mammary adenocarcinoma. The host response is manipulated in order to define cause-effect relationships. (2) characterization of the host responses that occur within the site of developing neoplasms and within tumor cell variants of distinct behavioral characteristics. The role of the tumor in evoking characteristic T cell and macrophage infiltrates is being elucidated. By focusing on the in situ aspects of host immunity, in the context of neoplastic progression, this application will contribute to rational development of the immunoprevention of cancer.